This invention relates to a solid bowl screw centrifuge comprising a cylindrical clarifying part, a conical demoisturizing part, a single flight or multiple flight, leading or trailing ribbon or solid blade screw, a suspension inflow opening into one end of the clarifying part or between the clarifying part and the demoisturizing part, a liquid discharge in the form of an overflow weir or stripping pipe on the clarifying part and a solids discharge on the demoisturizing part.
Centrifuges of this type (also known as decanters) are used in processing technology for separating the solid and liquid phase of suspensions. Separation is obtained by the solid being thrown outwards as sediment by the centrifugal force while in the conical/cylindrical rotating drum a screw rotating at a slightly different speed conveys the deposited solid to the discharge at the demoisturizing part. For further details, see the literature (e.g. Ullmanns Encyclopadie der Technischen Chemie, Volume I, 3rd Edition, 1951; F. Ch. Alt, W. Gosele, Einsatzkriterien fur Dekanter, Chem.-Ing.-Techn. 54 (1982)5, 425-430; W. Stahl, Th. Langeloh: Zur Verbesserung der Klarung in Dekantierzentrifugen, Chem.-Ing.-Techn. 55 (1983)4, 324-325; DE-OS 2 321 653; GM 1 760 883).
If the solid particles in suspensions sediment only slowly because they are very small or because there is very little difference between their density and that of the liquid, or if the solid sedimented from a suspension is thrown up again by the screw or by the overflow, then complete separation of the solid substance cannot be obtained at economical rates of throughput in a solid bowl screw centrifuge. There have long been attempts to improve the separation results, for example by modifying the transport of the suspension or liquid. Laminar fittings through which suspensions flow in an axial direction have been developed for the same purpose and/or the screw flights in the cylindrical clarifying part have been specially designed with this end in view (e.g. ribbon screws or screws with variable pitch). In spite of these measures, however, complete separation is in many cases impossible to achieve. It is at this point that the invention sets in.